Suspicion in the opposition is high that the killing was ordered by the Kremlin in retaliation for Nemtsov's adamant criticism of President Vladimir Putin. The 55-year-old was reportedly working on a report about Russian military involvement in the eastern Ukraine conflict.

But Russia's top investigative body said it was investigating several possible motives, including that he was killed in an attempt to smear Putin's image.

It also said it was looking into possible connections to Islamic extremism and Nemtsov's personal life.